Cast text to a double precision between -1.0 and 1.0 for use as a seed [postgresql]

I need to generate a random value using random() based on a seed which comes from an existing table. The seed is a text but setseed() requires a double between -1.0 and 1.0. The reason is that I need the random number to be the same given an existing value in my database.

I want to define a function that takes the seed and returns a random number based on that seed:

CREATE OR REPLACE FUNCTION random_(value text, salt text) RETURNS double precision AS
    $$
        DECLARE casted double precision;
        BEGIN
            -- cast value to a double between -1.0 and 1.0
            casted = ???

            setseed(casted);

            RETURN random();
        END
    $$ LANGUAGE plpgsql IMMUTABLE;

What would be the best way to transform the text into a valid argument for setseed()?